The invention relates to methods of forming finishes on thermoplastic articles and more particularly to a method of forming mouth adjacent cavities known as blowback cavities in glass vials and similar articles.
The production of small articles from thermoplastic materials, such as vials and ampoules from glass is a production task performed almost solely by automatic machines. Lengths of tubular glass preforms are heated, separated into appropriate lengths, sealed at one end and have an appropriate finish formed on the other. Machine art in this area is well developed and is exemplified by U.S. Pat. Nos. 3,171,730, 3,424,570, 3,792,991 and 4,080,189.
Whereas the formation of a reduced, uniform diameter opening in a vial may be readily fabricated by means of a smooth walled mandrel inserted thereinto, aluminum seal vials having syringe puncturable elastomeric inserts require the disposition of a large diameter cavity intermediate two smaller diameter regions to receive and retain the insert. Such a cavity is commonly denominated a blowback cavity. A correspondingly configured solid mandrel is obviously incapable of forming such an enlarged diameter cavity.
One prior art apparatus capable of fabricating such a cavity is disclosed in co-owned U.S. Pat. No. 3,202,495. Here, expandable mandrel segments are inserted into the mouth of a rotating vial and expanded to tool the desired cavity adjacent the mouth opening. When the tooling of the cavity has been completed, the mandrel segments are collapsed and removed from the vial. This method of fabrication is, however, somewhat slow in that the vial must be properly registered and rotated, the mandrel segments collapsed in order to enter the vial mouth, expanded to form the blowback cavity and finally collapsed in order to exit the vial.
As those generally familiar with machines which fabricate articles by a sequence of steps will readily appreciate, the overall fabrication rate is limited by the rate or time of the slowest step. In glass vial fabricating machines, this step typically has been associated with the formation of a blowback cavity. A method which both simplifies and speeds the formation of a blowback cavity in a vial or similar article would therefore be desirable.